I can confirm this.
Yesterday we encountered the Final Boss of the Foundry too.
We had the same Bug with the "Never-Ending"-Immunity Shield but the workaround from Auilt
Before a Lightning Storm we hit him down to 6% Life waited for the next Storm to finish and then "zerged" him down to 5%.
He casted his "Paladin-Bubble" sayed his last words an died.
Thanks again at Auilt
you saved our Evening an our Loot!
please fix the this Boss.
and yes, HK-47 is truely the most powerful Boss in this flashpoint. ^^